1. Field of the Invention
The present invention relates to an actuator used for a compact terminal device such as a mobile phone or the like.
2. Description of Related Art
Conventionally, an actuator of a bimorph or mono-morph type and so on has been developed. For example, there has been known an actuator of a mono-morph-type, which includes a holder and a beam whose central portion is adhesively fixed on the holder. The beam has a metallic plate referred to as a shim and piezoelectric layers formed on one surface of the shim. An alternating-current-signal voltage is applied to the piezoelectric layers in a direction of thickness thereof so as to expand and contract in a longitudinal direction of the piezoelectric layers, thereby flexural oscillation is excited to the beam.
There has been also known an actuator of a bimorph-type in which piezoelectric layers are formed on opposite surfaces of a metallic body.
There has been also known an actuator of a bimorph-type configured to hold by a hairpin-like resilient holding member a beam in which a plurality of laminated structures having piezoelectric and electrode layers are formed on opposite surfaces of a shim.
In these actuators, by adopting a structure in which homo-polar upper and lower electrode layers are connected with electrode layers extending along end surfaces of the piezoelectric layers, a wiring mechanism between the plurality of electrode layers and signal inputting terminals is simplified to intend reduction of a production cost and improvement in reliability due to the simplification of structure.
On the other hand, a user often drops accidentally a compact mobile instrument such as a mobile phone against a floor, for example. In this case, it is necessary to give a sufficient strength to the compact mobile instrument so that it is not broken.
If each of the above-mentioned actuators is installed in a mobile phone as a speaker, when dropping freely the mobile phone from a height of about 1.5 m against a concrete floor, it is necessary to have a sufficient strength so that the beam is not broken to the actuator for flexural oscillation of an excess amplitude generated in the beam by impact of the drop.
However, there is a problem that the beam is easy to be broken because the piezoelectric layers are made of brittle ceramic and the beam has an elongate shape and so on, in the above-mentioned flexural-oscillation-type actuator of the mono-morph or bimorph-type.
On the other hand, in the conventional bimorph-type actuator having the laminated structures formed on the both surfaces of the metallic body, there are problems that one or more sags which are unnecessary extending portions are formed on electrode layers extending along end surfaces of the piezoelectric layers in order to connect upper and lower homo-polar electrode layers, and the sags contact with the shim to generate a short circuit accident. To prevent the short circuit accident, a material for the shim must be changed to a resin or the like. This results in narrow flexibility for selecting the material.